1. Field of the Invention
The present invention relates to an image sensing apparatus such as a digital camera or a digital video camera, and to an image sensing system.
2. Description of the Related Art
With recent image sensing apparatuses such as digital cameras and digital video cameras, technology for reading out image signals from the image sensing element at high speed is required in order to increase pixel counts and improve continuous shooting speeds. In the case where signals read out at high speed are analog-to-digital (A/D) converted for every pixel, the conversion time per pixel data is extremely short. This requires extremely high-performance A/D converters capable of performing high-precision conversion in a short time, and is difficult to realize.
To solve this problem, research and development has been carried out into column analog-to-digital conversion (ADC) image sensing elements in which an A/D converter is disposed for every pixel column using CMOS image sensors that can be manufactured with a similar process to CMOS integrated circuits, and simultaneously performing A/D conversion on the respective pixel signals of one row of pixels. Column ADC image sensing elements are advantageous in that the readout rate of the image sensing element for a full image (one screen) can readily be increased, since the conversion rate of the A/D converters can be reduced from the readout rate of one pixel to the readout rate of one row.
Examples of such a column ADC image sensing element include an image sensor using ramp ADCs that apply a triangular wave (see Japanese Patent Laid-Open No. 05-048460) and an image sensor using successive approximation ADCs (see U.S. Pat. No. 5,880,691).
On the other hand, there are cases in which long exposures are made particularly in still photography. With this type of shooting, the charge accumulation period of the image sensing element is also lengthened, and dark current accumulates in the photodiode. Since this dark current component is read out together with the optical signal component when the pixel signals are read out, the dynamic range is compressed by the amount of dark current when A/D conversion is performed.
With a configuration in which one or more A/D converters are externally connected to the image sensing element in place of the column ADC system, an analog front end (AFE) having a programmable gain amplifier (PGA) function and an optical black (OB) clamp function in addition to the A/D conversion function is typically used. The offset of the image sensing element including dark current and the offset of the PGA and the A/D converters are removed using the OB clamp function. The OB clamp function adjusts the black level that includes dark current to a desired value, securing the dynamic range of the signals since A/D conversion is performed after the dark current component has thereby been eliminated.
In terms of circuit configurations for realizing this OB clamp function, there is disclosed a configuration utilizing an integrating circuit that uses a capacitor (see Japanese Patent Laid-Open No. 5-153428) and a configuration using a digital-to-analog (D/A) converter (see Japanese Patent Laid-Open No. 2000-224440).
Since the column ADC system is designed for reading out the signals of the image sensing element at high speed, it is normally premised on short exposures, which limits the dark current component produced per frame. Consequently, the dynamic range is not so adversely affected even when A/D conversion is performed without removing the dark current component. However, when application of this system to digital cameras and digital video cameras capable of also performing long exposures is considered, the removal of the dark current component is essential to securing the dynamic range.
However, since the column ADC system requires that A/D converters equal in number to the pixel columns be formed on the same semiconductor chip as the image sensing element, it is extremely difficult in terms of chip size to configure an OB clamp circuit for the A/D converter of each column.
Further, even assuming an OB clamp circuit is configured for each column, only the shading pixels in the columns can be utilized in OB clamping. The number of shading pixels provided per column is limited by chip size, this number being insufficient as information for use in OB clamping to sufficiently remove noise.